We have designed a novel photon-counting, energy discriminating x-ray detector; we hypothesize that our design can provide limiting resolution on the order of 5 microns, while also providing very high x-ray detection efficiency and adequate count rates for 25 keV photons. The basic detector unit is inexpensive and simple to manufacture, as well as radiation hard. The complexity of the detection task is shifted to the high-speed electronics and data acquisition system, which will process every single photon-created charge cloud to determine the photon's energy and position of interaction. The design could be implemented using one of several different semiconductor materials, thereby enabling clinical applications at x-ray energies ranging from 15 keV to 50 keV. This project has the overall goal of providing the tools for optimization of the detector, as well as demonstrating that ultra-high resolution can be achieved using a prototype implemented in Silicon for detection of 25 keV photons. We will build a numerical model of the detector by combining three components: 1. a Monte-Carlo simulation of photon interactions in Si; 2. a detailed description of energy-loss mechanisms for electrons to provide complete knowledge of initial charge cloud distribution in Si; and 3. a model of bias field distribution and resulting path of charges through the Si. Combining the three components permits prediction of measured signal for a given detector geometry, applied bias field and readout electronics design. We will also build a first (somewhat sub-optimal) prototype of the detector to permit validation of the model, and to permit verification of the hypothesis regarding achievable resolution. The final research product will be an accurate and validated numerical model of the Si detector, which can then be used to optimize the geometry and electronics design for a pre-clinical (or breast imaging) version of the detector. Our initial clinical goal is to enable novel x-ray phase contrast imaging systems (both propagation-based and grating-based) that are both simple to implement and dose efficient - hence the need for high resolution and high quantum detection efficiency. However, since our design is photon counting and energy-discriminating, the same basic detector design would also provide improved dose efficiency (on the order of 15% to 60% depending on the imaging task in CT) as well as being applicable to other novel imaging tasks such as k- and l-shell fluorescence imaging.